Diffraction gratings are used in spectrographs and other applications to diffract light and separate light into component colors or wavelengths. One conventional type of diffraction grating is a ruled or surface relief grating, which may be transmissive or non-transmissive, and which diffracts light using grooves or other surface features. One problem with ruled gratings is that the grooves or surface features have to be mechanically machined in the surface of the grating and maintaining constant spacing between grooves or surface features is difficult. This can be addressed by well-known methods to optically record and etch features in the surface, but achieving high efficiency requires control of the shape of the grooves, which is difficult by etching.
Another type of grating used in spectrographs is a phase grating. Phase gratings are transmissive and can be more efficient than non-transmissive gratings. Phase gratings incorporate structures that diffract light by changing its phase as it passes through the grating. Phase gratings are easier to manufacture than non-transmissive gratings because the holographic fringes that produce the diffracting structures can be produced using interfering laser beams and recorded as refractive index changes in a variety of transparent media.
It is desirable to have phase gratings that are curved or spherically shaped for some advantageous spectrograph designs. However, recording fringes in spherical phase gratings for some applications is difficult, especially when the application requires careful control of the fringe tilt angle and the spacing between fringes.
Accordingly, there exists a need for a curved VPH diffraction grating with tilted fringes and spectrographs that use such a grating.